To increase the payload mass delivered to orbit, large geosynchronous spacecraft may perform an orbit transfer using high-efficiency ion propulsion. During the orbit transfer, either Hall Current Thrusters (HCTs) or Xenon Ion Thrusters (XITs) are fired nearly continuously for a period of several weeks to months. While the thrusters are fired, the spacecraft attitude is controlled so the thrust vector tracks a specified inertial trajectory vector and the solar arrays remain pointed at the sun. The orbit transfer trajectory vector may be determined by solving the well-known minimum-time continuous firing orbit transfer problem. The trajectory is determined using a computationally intensive numerical iteration procedure that is not practical to implement on-board the spacecraft. The results of the optimization are a set of parameters, known as co-states, that can be used to generate the thrust trajectory on board the spacecraft.
During actual mission operations, the spacecraft generates the orbit transfer trajectory vector using ground-supplied co-states, and follows the trajectory while firing ion thrusters to execute the orbit transfer. However, over time, the actual orbit trajectory will deviate from the ideal orbit trajectory due to the effects of thrust and mass uncertainties, attitude determination and control errors, and perturbations due to the sun, moon, and earth gravity non-uniformities. To mitigate these effects, the ground routinely performs orbit determination using ranging data collected at times when ground contact with the spacecraft is possible. Using the estimated orbit, the ground generates updated co-states that are sent to the spacecraft to compute the updated thrust trajectory that corrects the orbit. This trajectory correction procedure is repeated every several days until the orbit transfer is complete.
There are several significant drawbacks to this approach. The first is that it requires a potentially large and highly skilled ground support staff to generate the orbit determination and trajectory replans for upload to the spacecraft. This staff must support the entire orbit transfer, which can be 100 days or greater. Also, because of the need for ground contact for ranging and commanding, generally at least three ground stations widely spaced in longitude must be available. Ground stations and their support staffs are a significant mission support cost, and it is desirable to reduce the number of stations needed. Additionally, there is a practical limit to the frequency that the firing plan can be updated due to the time required to collect and process orbit determination data and perform the numerical optimization. Larger update intervals reduce the orbit transfer efficiency, because the actual trajectory will deviate to a greater extent from the ideal trajectory. This deviation results in a longer orbit transfer requiring more fuel to complete.
Finally, the upload plans and parameters must be carefully checked and verified by the ground support staff before transmission to the spacecraft. As is true with all operations involving frequent ground commanding, mistakes are possible that can have negative consequences for the mission.